Thanks for the interesting piece on pressurized versus pressure-less balls. I’ve ordered a batch of the pressure-less, and am curious to give them a try. On a similar wavelength, I’d like to broach the topic of high-altitude balls. What’s the problem with using regular pressurized balls at high altitudes? Why are high-altitude balls better suited to the conditions?—Thomas U.

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The problem, Thomas, is that pressurized balls are much livelier at altitude—i.e., they move faster and bounce higher—which can make for extremely erratic play. (The International Tennis Federation considers “high altitudes” to be 4000 feet or more above sea level.) As a corrective, several manufacturers produce special balls that are bigger in circumference, less pressurized, or pressure-less. These changes in fabrication reduce the balls’ liveliness, normalizing their behavior in high-altitude conditions.

(Fact: Pressure-less balls perform consistently regardless of sea level; however, ITF rules state that they must be “acclimatised for 60 days or more at the altitude of the specific tournament” before use.)

That’s the short answer. The more descriptive, technical answer has to do with physics of air masses. Why is the ball’s behavior largely a function of its altitude, anyways? There are two major reasons.

Unsurprisingly, the first involves pressure. As I explained last week, the common tennis ball is manufactured with an internal air pressure higher than that of the ambient atmosphere. This pressure differential is, per ITF rules, standardized across brands, and for good reason: It’s a large factor in how high the ball bounces and how stiff it feels.

But at higher altitudes, explains Howard Brody, in The Physics and Technology of Tennis, “the outside, atmospheric pressure is reduced. This results in an increase in the…difference between the internal pressure of a normal tennis ball and atmospheric pressure. The larger this difference in pressure, the greater…[the effect will be on] both the bounce of the ball on the court (it will bounce higher) and the speed of the ball as it comes off of the strings (it will give you more power).”

The second reason has to do with the thinness of the air. “The flight of the ball,” continues Brody, “is influenced by the force of air resistance. The more air molecules the ball runs into, the more the ball will slow down. At high altitude, the ball will run into less air molecules…so it will not slow down as much.”